1. Field of the Invention
The present invention relates to apparatus of use with magnetic ink character recognition systems for determining whether character defining magnetic material is present or absent in a particular space of interest. The invention relates specifically to means for distinguishing between intercharacter gaps and certain portions of magnetic characters.
2. Description of the Prior Art
Among the prior art characters employed in magnetic ink character reading systems are the MICR E13B characters formed as a 7.times.9 matrix which, in a standard format, are made up of squares of a standard size which are referred to hereinafter as feature periods.
It is impossible to assure absolutely uniform distribution of magnetic ink on paper documents such as bank checks. Every irregularity in the fibre of the paper results in variations in the amount of ink registered on the paper. In addition, variations in the amount of ink applied by the printing press adds to this lack of uniformity. As a consequence, the output signals from a read head as it passes over areas bearing magnetic ink include many irregularities caused by changes in magnetic ink density, voids in the ink, irregular edges on the characters and the like. In the past, attempts to compensate for these irregularities have led to the use of imposed a.c. fields and filters, as indicated below.
The signal, or waveform, generated as a character formed of magnetic material on the face of a document is moved past a read head is proportional to the first derivative of the flux distribution associated with the magnetic material. For reasons given above, this signal includes many irregularities. A common practice in the past has been to clean up this signal, or waveform, by passing it through a low pass filter. The low pass filter strips off the noise components generated by variations in the character edges, the fringing at the head gap, voids in the magnetic ink, variations in the magnetic ink density and the like.
An unfortunate consequence of filtering signals in the manner described is a loss of information, making it more difficult to distinguish between the intercharacter gaps and certain portions, or feature periods, of magnetic characters. The feature periods which are most difficult to distinguish from intercharacter gaps are periods from which the read head generates relatively low signals due to the fact that the amount of magnetic material sensed remains relatively constant throughout the period. Examples of such feature periods are found wherever part of a character is formed of magnetic material located only in horizontal patterns between parallel horizontal edges as opposed to areas where magnetic material is found in vertical patterns which cause wide variations in signals produced by the read head as it passes.
A prior art means used to distinguish between the intercharacter gaps and the magnetic characters present has been to magnetize the magnetic material of the characters with an a.c. field. The read head then senses both the a.c. field and the d.c. field, thus better enabling the system to detect the presence of material forming parts of characters which produce low amplitude signals as distinct from the absence of material indicating intercharacter spaces. An objection to this prior art procedure is that an additional write head to inject the a.c. field and an a.c. oscillator to drive the additional head are required.
The present invention avoids the disadvantages of the prior art by making use of irregularities in the signal derived from the read head as it passes over the printed characters. In this regard, it has been discovered that the irregularities in the signal as the head passes over individual magnetic characters, which in many respects correspond to an a.c. bias, may be used to establish the presence of magnetic material forming a character when contrasted to the general absence of irregularities in the signal as the read head passes over an intercharacter gap. This distinction is particularly important in the prevention of an erroneous indication of an intercharacter gap in certain portions of characters having horizontal areas which incorporate a relatively constant amount of magnetic material. An example is the character zero which includes horizontal spaces, or feature periods, in which the only variations in signal strength produced by the read head are the consequence of accidental variations in the amount of magnetic material deposited between edges forming the tops and bottoms, or upper and lower boundaries, of horizontal portions of the zero.